Patients with rheumatic or connective tissue disease such as systemic lupus erythematosus, diffuse systemic sclerosis, CREST syndrome, mixed connective tissue disease, rheumatoid arthritis, and other autoimmune diseases produce antibodies against nuclear antigen. Many of these antibodies are diagnostic, and others by their presence exclude specific similar but not easily distinguishable disease entities. The molecular identities of only the most common antigens have been determined and several different diagnostic tests are available to identify them. They are specific only by comparison with reference sera. We will develop recombinant antigens with defined sera from human cDNA-derived expression libraries. Recombinant proteins with the respective epitopes will be assembled in a diagnostic kit that allows unambiguous identification of the actual antigen in a single simple immunoblot test. The tests will have a specificity not obtainable in present diagnostic laboratories and will be constructed so that it is easily readable excluding the judgemental analysis prevalent in present techniques.